Difluoromethane (known in the trade under the designation F32) is one of the possible substitutes for chlorofluorocarbons (CFCs) which are dealt with by the Montreal Protocol and is intended to replace more particularly chloropentafluoroethane (F115), the action of which on ozone is accompanied by a powerful contribution to the greenhouse effect.
F32 can be obtained by fluorination of methylene chloride (CH.sub.2 Cl.sub.2) by means of HF in the presence of a catalyst or by hydrogenolysis of dichlorodifluoromethane (F12) or of chlorodifluoromethane (F22) or else by decomposition, in the presence of HF, of alpha-fluoroethers under the action of Lewis acids.
However, fluorination processes exhibit the disadvantage of involving the formation, as intermediate, of chlorofluoromethane (F31), a toxic compound which, according to the classification of the IARC (International Agency for Research on Cancer) is classified in category IIB (possibly cancerogenic to man) and the residual concentration of which it is appropriate to lower to about one ppm. Despite the difference in volatility between F31 (b.p. -9.1.degree. C.) and F32 (b.p.: -51.7.degree. C.), this objective is difficult to obtain by distillation and therefore requires the development of a highly efficient purification process. Small quantities of F31 may also be present in an F32 manufactured by hydrogenolysis of F12 or of F22.
To lower the F31 content in an F32, patent application EP 508 630 describes a process consisting in placing the F32 to be purified in contact with an activated carbon. The selectivity is not very high and the methylene chloride or the dichlorodifluoromethane is adsorbed in the same proportions as F31, thus limiting the capacity of the adsorbent. It is known, furthermore, that the properties of an activated carbon depend greatly on its method of preparation and on the raw material employed; the effectiveness of an activated carbon and above all its selectivity are therefore liable to variations depending on the source of the batches.
The use of molecular sieves for the purification of fluorohydrocarbons is already known. The purification treatments are usually performed at ambient temperature or thereabouts. Temperatures lower than ambient temperature are sometimes recommended, as in U.S. Pat. No. 2,917,556, claiming the use of 5A, 10X or 13X sieves for the removal of vinyl fluoride from vinylidene fluoride or in patent application JP 5-70381, which recommends a temperature region ranging from -30.degree. to +30.degree. C. for removing HFC 365 (C.sub.4 F.sub.5 H.sub.5) and the corresponding olefins (C.sub.4 F.sub.4 H.sub.4) from a crude 1,1-dichloro-l-fluoroethane (F141b) by treatment on 13X molecular sieve.
With zeolites A or a natural sieve such as chabazite, patent application EP 503 796 recommends temperatures of between +10.degree. and +100.degree. C. for removing any trace of 1-chloro-2,2-difluoroethylene (F1122) from 1,1,1,2-tetrafluoroethane (F134a), but the treatments are in practice performed at 40.degree. C.
In the purification of F 134a, but for the removal of 1,1,2,2-tetrafluoroethane (F134), patent application JP 3-72437 employs zeolites which have a pore diameter of between 5 and 7 .ANG. over a temperature region of between 0.degree. and 70.degree. C.
These sieves are generally regenerated by heating to 200.degree.-350.degree. C. under a stream of air or nitrogen, by heating under reduced pressure or else by displacement of the adsorbed products with water and reactivation at high temperature, as indicated in U.S. Pat. No. 2,917,556. These regeneration processes, well known to a person skilled in the art, appear in most of the technical data sheets supplied by the manufacturers of molecular sieves.
Although patent DE 1 218 998 mentions that 13X molecular sieves can work at a relatively high temperature, none of the abovementioned documents allows any change or increase in selectivity whatever to be expected when the temperature of treatment is raised.